The present invention relates generally to measuring characteristics of a web of moving sheet material and, more particularly, to a number of improved methods and apparatus which individually and cooperatively enhance performance of such measurements. For example, an easily accessible radiation source may be mounted in a rotary shutter for generating a measuring radiation beam and providing a short air gap to an associated radiation detector. The radiation beam may be tuned for improved measuring performance and may be fan-shaped and sized relative to the radiation detector such that the ends of the beam overlap the detector with the system being tuned for improved performance. The improvements of the present invention provide superior streak detection, composition and alignment insensitivity, reliability, accuracy and stability.
Systems for measuring characteristics of moving webs of a wide variety of sheet materials are well known in the art. For example, characteristics are commonly measured by scanning a measurement head transversely back and forth across a moving web. The measurement head includes a source of radiation which is supported on one side of the web of material, and a detector for sensing the radiation supported directly opposite to the source. The radiation is attenuated by the web of sheet material such that the amount of radiation received by the detector is representative of a characteristic which is to be measured. The type of radiation may be selected dependent upon the material making up the web and the characteristic or characteristics to be measured.
While prior art systems have been effectively used for a substantial period of time, the requirements for the performance of these systems has been constantly increasing. As an example, in the paper making industry, the specifications for paper making process variability have been tightened by at least an order of magnitude over the last twenty years. The increasing performance standards require improved sensors, systems employing the sensors and automatic control techniques which remove a substantial portion of error inherent in subjective human control.
The task of meeting the constantly advancing requirements has been exacerbated by a corresponding trend towards operating web making processes at higher temperatures, higher speeds and often with more recycled and dusty materials. All of these operating trends contribute to make an already hostile environment for web measurement systems even more hostile.
Improving the various aspects of web measurement systems has been difficult. For example, if a basis weight sensor is designed for improved performance in one area, such as composition insensitivity, the design "improvement" may cause the system to be more noise or alignment sensitive. These design problems are due to the physics of radiation interaction with the materials making up the webs which are to be measured.
It is thus apparent that there is an ongoing need for improved systems for measuring web characteristics. Preferably, improvements to web measuring systems could be combined to provide overall improved measurement and also be individually applicable for overcoming specific measurement problems.